Three prior art low input capacitance amplifiers are the P6051 and P6201 oscilloscope probes of Tektronix Corporation, Beaverton, Oreg. and the LH0002/LH0002C general purpose thick film hybrid current operational amplifier of National Semiconductor Corporation, Santa Clara, Calif. Although considerably different in electrical design, number of components, and physical size from the current invention, the concept of the Tektronix P6201 probe will be discussed as indicative of the sophisticated and complex prior art resulting in a rated input capacitance of approximately 1.5 picofarads when shunted by 1.0 megohm attenuator heads. This abbreviated description is but a condensation and paraphrase of the more complete and exacting description of this circuit as appears in the instrution manual for this device.
As an active (FET) probe providing unity gain from DC to 900 MHz bandwidth, the low capacitance of the P6201 probe permits coupling of such high frequency signals to an oscilloscope input with minimum loading of the circuit under test. The P6201 Probe system uses a DC reinsertion amplifier technique to obtain low drift and broad-band performance. The input signal is simultaneously coupled in parallel paths to an AC/High Frequency Amplifier and to a DC/Low Frequency Amplifier. Signal outputs from these two stages are recombined in an Output Amplifier which provides the output signal at the proper 50 ohm load impedance. The AC/High Frequency Amplifier stage which is most comparable to the circuit of the present invention is based on an input FET operated as a source follower, such as provides the high input-resistance and low-noise performance of the probe input. No feedback loops concerning this input FET are utilized. Subsequent to the input FET there are emitter follower stages which provide the current gain and impedance isolation between the input signal and the output signal, in this case the output signal to the further Output Amplifier stage. Net 1.5 picofarad input capacitance of the P6201 probe amplifier is effectively resultant from the junction to junction, junction to case, and parasitic capacitances surrounding all signal carrying components within the circuit of the source follower operated input FET.